With wide varieties of bands used recently in wireless communications with cell phones, etc., cell phones capable of handling pluralities of transmission/reception bands for a dual band system, a triple band system, a quadband system, etc. are widely used. For example, the frequency bands for communications systems used in quadband cell phones include GSM850/900 (824-960 MHz), DCS (1710-1850 MHz), PCS (1850-1990 MHz), UMTS (Band1: 1920-2170 MHz, Band2: 1850-1990 MHz, Band3: 1710-1880 MHz, Band4: 1710-2155 MHz, Band5: 824-894 MHz, Band6: 830-885 MHz, Band1: 2500-2690 MHz, Band8: 880-960 MHz, Band9: 1749.9-1879.9 MHz, Band10: 1710-2170 MHz), etc. For example, the frequencies of DCS (digital cellular system), PCS (personal communication services) and UMTS Band1 (universal mobile telecommunications system) are substantially 2-2.5 times the frequency of GSM (global system for mobile communications). In the following explanation, GSM is used in a basic frequency band, and frequency bands higher than it are called higher frequency bands.
As communications apparatuses such as multiband cell phones, etc. have become smaller and lighter with higher performance, switch circuits constituting high-frequency circuits contained therein have had a larger number of ports, resulting in many switch circuits with 4 or more ports. JP 2005-123740 A discloses a high-frequency switch module shown in FIG. 26, which comprises a single-pole, five-throw switch circuit having FET switch circuits. This high-frequency switch module 20 for use in triple band cell phones comprises a switch device 10 comprising a single-pole, five-throw switch circuit 10a and a decoder 10b, lowpass filter circuits 15a, 15b, bandpass filter circuits 15c, 15d, 15e, and an ESD protection circuit 18. FIG. 27 shows the structure of the single-pole, five-throw switch circuit 10a. The single-pole, five-throw switch circuit 10a, which is obtained by parallel-connecting single-pole, single-throw FET switch circuits having the same circuit structure, comprises a common terminal a to be connected to an antenna ANT, and transmission terminals b, c and receiving terminals d, e, f to be connected to transmission/reception circuits, etc.
FIG. 28 shows the structure of an FET switch circuit Q1 as an example of FET switch circuits. The FET switch circuit Q1 comprises a terminal 250a for inputting signals, a terminal 250b for outputting signals, and a control terminal Vc1 to which voltage for ON/OFF control is applied. A resistor R is connected to block current leakage. This example comprises one FET, but two or more FETs may be connected in series to reduce equivalent capacitance at OFF, thereby preventing harmonic distortion.
To improve isolation characteristics between the ports, the FET switch circuit may comprise FET Q1-1 connected to the input terminal 250a and the output terminal 250b, and shunt-connected FET Q1-2 (FIG. 31). Connected to the gates of FETs Q1-1 and Q1-2 each via a resistor R for blocking current leakage are control terminals Vc1, Vc2 applying voltage for controlling the ON/OFF state. When the input terminal 250a and the output terminal 250b are connected, Q1-1 is in an ON state, while Q1-2 is in an OFF state (FIG. 32). On the other hand, when the input terminal 250a and the output terminal 250b are in a disconnected state, Q1-1 is in an OFF state, while Q1-2 is in an ON state (FIG. 33). In any state, the equivalent capacitance of Q1-1 or Q1-2 is connected to the input terminal 250a, resulting in more equivalent capacitance added than the FET switch circuit shown in FIG. 28.
The single-pole, multi-throw switch circuit 10a may be constituted by combining different FET switch circuits. In that case, for example, the FET switch circuit shown in FIG. 28 may be used in some signal paths, and the FET switch circuit shown in FIG. 31 may be used in the other signal paths.
FIG. 29 shows the equivalent circuit of the single-pole, five-throw switch circuit 10a, in which the common terminal a and the transmission terminal b are in a connected state, and FIG. 30 shows the equivalent circuit when the equivalent capacitance in an OFF state is synthesized. The FET switch is equivalent to the resistor R when it is in an ON state, and equivalent to capacitance Coff when it is in an OFF state. In the single-pole, multi-throw switch circuit, an FET switch in one of the signal paths is controlled to an ON state, so that the common terminal a is connected to one of the transmission terminals b, c and receiving terminals d e, f. The transmission terminals b, c and receiving terminals d, e, fare connected to load impedances Zlb-Zlf.
More parallel-connected FET switch circuits provide more capacitance Coff added between the single-pole, multi-throw switch circuit and the ground, so that the impedance of the single-pole, multi-throw switch circuit is capacitive when viewing the transmission terminal b from the common terminal a. With the increased capacitance Coff, impedance changes lower along an equal conductance line in a Smith chart, resulting in lower return loss and increased insertion loss. This is a problem to be overcome for single-pole, multi-throw switch circuits with an increasingly larger number of ports (JP 8-223021 A).
To solve this problem, JP 2008-124556 A proposes the series connection of inductors to ports at a common terminal and other terminals for impedance matching. However, JP 2008-124556 A does not investigate at all a case where different equivalent capacitances are provided to signal paths by FET switch circuits.
Wireless communications apparatuses such as cell phones, etc. are operated not only in different frequency bands but also with different powers depending on the communications systems. For example, the maximum transmission power of GSM850/900 is larger than those of the other communications systems, and DCS, PCS and UMTS conduct transmission and reception with different powers. Correspondingly, FET switches have different structures, such as gate width adjusted for every signal path, the parallel connection of capacitance between the gate and the source or drain, etc. Accordingly, the equivalent capacitance of the FET switch circuit also differs for every signal path, resulting in impedance variation (deviation) due to the equivalent capacitance, which also differs for every signal path. Its influence is larger in higher frequency bands such as PCS and UMTS than the basic frequency band such as GSM, etc.
JP 2008-124556 A describes that when the standard of return loss differing for every signal path is required, the return loss can be finely adjusted by the position of the inductor. However, when the inductor is constituted by a bonding wire, pluralities of bonding wires are crossing and nearing, generating interference between signal paths, resulting in the deterioration of isolation characteristics and impedance difference due to the parasitic reactance. Transistors and inductors can be integrated on a silicon substrate to provide IPDs (integrated passive devices), but it makes the single-pole, multi-throw switch circuit larger, and provides such problems as the deterioration of isolation characteristics and impedance difference due to the parasitic reactance.